63 research outputs found
Highly Quantum-Confined InAs Nanoscale Membranes
Nanoscale size-effects drastically alter the fundamental properties of
semiconductors. Here, we investigate the dominant role of quantum confinement
in the field-effect device properties of free-standing InAs nanomembranes with
varied thicknesses of 5-50 nm. First, optical absorption studies are performed
by transferring InAs "quantum membranes" (QMs) onto transparent substrates,
from which the quantized sub-bands are directly visualized. These sub-bands
determine the contact resistance of the system with the experimental values
consistent with the expected number of quantum transport modes available for a
given thickness. Finally, the effective electron mobility of InAs QMs is shown
to exhibit anomalous field- and thickness-dependences that are in distinct
contrast to the conventional MOSFET models, arising from the strong quantum
confinement of carriers. The results provide an important advance towards
establishing the fundamental device physics of 2-D semiconductors
Capture rate and neutron helicity asymmetry for ordinary muon capture on hydrogen
Applying heavy-baryon chiral perturbation theory to ordinary muon capture
(OMC) on a proton, we calculate the capture rate and neutron helicity asymmetry
up to next-to-next-to-leading order. For the singlet hyperfine state, we obtain
the capture rate Gamma_0 = 695 sec^{-1} while, for the triplet hyperfine state,
we obtain the capture rate Gamma_1 = 11.9 sec^{-1} and the neutron asymmetry
alpha_1 = 0.93. If the existing formalism is used to relate these atomic
capture rates to Gamma_{liq}, the OMC rate in liquid hydrogen, then Gamma_{liq}
corresponding to our improved values of Gamma_0 and Gamma_1 is found to be
significantly larger than the experimental value, primarily due to the updated
larger value of g_A. We argue that this apparent difficulity may be correlated
to the specious anomaly recently reported for mu^- + p to n + nu_mu + gamma,
and we suggest a possibility to remove these two "problems" simply and
simultaneously by reexamining the molecular physics input that underlies the
conventional analysis of Gamma_{liq}.Comment: 14 pages, 1 figur
Ultrathin compound semiconductor on insulator layers for high performance nanoscale transistors
Over the past several years, the inherent scaling limitations of electron
devices have fueled the exploration of high carrier mobility semiconductors as
a Si replacement to further enhance the device performance. In particular,
compound semiconductors heterogeneously integrated on Si substrates have been
actively studied, combining the high mobility of III-V semiconductors and the
well-established, low cost processing of Si technology. This integration,
however, presents significant challenges. Conventionally, heteroepitaxial
growth of complex multilayers on Si has been explored. Besides complexity, high
defect densities and junction leakage currents present limitations in the
approach. Motivated by this challenge, here we utilize an epitaxial transfer
method for the integration of ultrathin layers of single-crystalline InAs on
Si/SiO2 substrates. As a parallel to silicon-on-insulator (SOI) technology14,we
use the abbreviation "XOI" to represent our compound semiconductor-on-insulator
platform. Through experiments and simulation, the electrical properties of InAs
XOI transistors are explored, elucidating the critical role of quantum
confinement in the transport properties of ultrathin XOI layers. Importantly, a
high quality InAs/dielectric interface is obtained by the use of a novel
thermally grown interfacial InAsOx layer (~1 nm thick). The fabricated FETs
exhibit an impressive peak transconductance of ~1.6 mS/{\mu}m at VDS=0.5V with
ON/OFF current ratio of greater than 10,000 and a subthreshold swing of 107-150
mV/decade for a channel length of ~0.5 {\mu}m
Effective Field Theory for Low-Energy Two-Nucleon Systems
We illustrate how effective field theories work in nuclear physics by using
an effective Lagrangian in which all other degrees of freedom than the
nucleonic one have been integrated out to calculate the low-energy properties
of two-nucleon systems, viz, the deuteron properties, the np 1S0 scattering
amplitude and the M1 transition amplitude entering into the radiative np
capture process. Exploiting a finite cut-off regularization procedure, we find
all the two-nucleon low-energy properties to be accurately described with
little cut-off dependence, in consistency with the general philosophy of
effective field theories.Comment: 4 pages, RevTeX, two eps files. Slightly modified. To appear in Rapid
Communication Section of Phys. Rev.
Coincidence analysis to search for inspiraling compact binaries using TAMA300 and LISM data
Japanese laser interferometric gravitational wave detectors, TAMA300 and
LISM, performed a coincident observation during 2001. We perform a coincidence
analysis to search for inspiraling compact binaries. The length of data used
for the coincidence analysis is 275 hours when both TAMA300 and LISM detectors
are operated simultaneously. TAMA300 and LISM data are analyzed by matched
filtering, and candidates for gravitational wave events are obtained. If there
is a true gravitational wave signal, it should appear in both data of detectors
with consistent waveforms characterized by masses of stars, amplitude of the
signal, the coalescence time and so on. We introduce a set of coincidence
conditions of the parameters, and search for coincident events. This procedure
reduces the number of fake events considerably, by a factor
compared with the number of fake events in single detector analysis. We find
that the number of events after imposing the coincidence conditions is
consistent with the number of accidental coincidences produced purely by noise.
We thus find no evidence of gravitational wave signals. We obtain an upper
limit of 0.046 /hours (CL ) to the Galactic event rate within 1kpc from
the Earth. The method used in this paper can be applied straightforwardly to
the case of coincidence observations with more than two detectors with
arbitrary arm directions.Comment: 28 pages, 17 figures, Replaced with the version to be published in
Physical Review
Results of the search for inspiraling compact star binaries from TAMA300's observation in 2000-2004
We analyze the data of TAMA300 detector to search for gravitational waves
from inspiraling compact star binaries with masses of the component stars in
the range 1-3Msolar. In this analysis, 2705 hours of data, taken during the
years 2000-2004, are used for the event search. We combine the results of
different observation runs, and obtained a single upper limit on the rate of
the coalescence of compact binaries in our Galaxy of 20 per year at a 90%
confidence level. In this upper limit, the effect of various systematic errors
such like the uncertainty of the background estimation and the calibration of
the detector's sensitivity are included.Comment: 8 pages, 4 Postscript figures, uses revtex4.sty The author list was
correcte
Observation results by the TAMA300 detector on gravitational wave bursts from stellar-core collapses
We present data-analysis schemes and results of observations with the TAMA300
gravitational-wave detector, targeting burst signals from stellar-core collapse
events. In analyses for burst gravitational waves, the detection and
fake-reduction schemes are different from well-investigated ones for a
chirp-wave analysis, because precise waveform templates are not available. We
used an excess-power filter for the extraction of gravitational-wave
candidates, and developed two methods for the reduction of fake events caused
by non-stationary noises of the detector. These analysis schemes were applied
to real data from the TAMA300 interferometric gravitational wave detector. As a
result, fake events were reduced by a factor of about 1000 in the best cases.
The resultant event candidates were interpreted from an astronomical viewpoint.
We set an upper limit of 2.2x10^3 events/sec on the burst gravitational-wave
event rate in our Galaxy with a confidence level of 90%. This work sets a
milestone and prospects on the search for burst gravitational waves, by
establishing an analysis scheme for the observation data from an
interferometric gravitational wave detector
Dramatic reduction of surface recombination by in-situ surface passivation of silicon nanowires
Nanowires have unique optical properties [1-4] and are considered as
important building blocks for energy harvesting applications such as solar
cells. [2, 5-8] However, due to their large surface-to-volume ratios, the
recombination of charge carriers through surface states reduces the carrier
diffusion lengths in nanowires a few orders of magnitude,[9] often resulting in
the low efficiency (a few percent or less) of nanowire-based solar cells. [7,
8, 10, 11] Reducing the recombination by surface passivation is crucial for the
realization of high performance nanosized optoelectronic devices, but remains
largely unexplored. [7, 12-14] Here we show that a thin layer of amorphous
silicon (a-Si) coated on a single-crystalline silicon nanowire (sc-SiNW),
forming a core-shell structure in-situ in the vapor-liquid-solid (VLS) process,
reduces the surface recombination nearly two orders of magnitude. Under
illumination of modulated light, we measure a greater than 90-fold improvement
in the photosensitivity of individual core-shell nanowires, compared to regular
nanowires without shell. Simulations of the optical absorption of the nanowires
indicate that the strong absorption of the a-Si shell contributes to this
effect, but we conclude that the effect is mainly due to the enhanced carrier
lifetime by surface passivation
Learning the Optimal Control of Coordinated Eye and Head Movements
Various optimality principles have been proposed to explain the characteristics of coordinated eye and head movements during visual orienting behavior. At the same time, researchers have suggested several neural models to underly the generation of saccades, but these do not include online learning as a mechanism of optimization. Here, we suggest an open-loop neural controller with a local adaptation mechanism that minimizes a proposed cost function. Simulations show that the characteristics of coordinated eye and head movements generated by this model match the experimental data in many aspects, including the relationship between amplitude, duration and peak velocity in head-restrained and the relative contribution of eye and head to the total gaze shift in head-free conditions. Our model is a first step towards bringing together an optimality principle and an incremental local learning mechanism into a unified control scheme for coordinated eye and head movements
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